To produce white light from a solid state device, designers have tried encapsulating blue LEDs with a yellow fluorescent layer. For example, U.S. Pat. No. 5,998,925 to Shimizu et al., issued Dec. 7, 1999 and entitled “Light Emitting Device Having a Nitride Compound Semiconductor and a Phosphor Containing a Garnet Fluorescent Material,” teaches such a light-emitting diode 100 as illustrated in FIG. 1. LED 100 is a through-hole light-emitting diode having a mount lead 105 and an inner lead 106, wherein a light-emitting component 102 is installed on a cup 105a of the mount lead 105, and the cup 105a is filled with a coating resin 101 which contains a specified phosphor to cover the light-emitting component 102. An n-electrode and a p-electrode of the light-emitting component 102 are connected to the mount lead 105 and the inner lead 106, respectively, by means of wires 103.
When the LED 100 is forward-biased, a portion of the light emitted by the light-emitting component 102 stimulates the phosphor contained in the coating resin 101 to generate fluorescent light having a wavelength different from that of the LED light. In this manner, the fluorescent light emitted by the phosphor and the LED light that is output without contributing to the excitation of the phosphor are mixed and output. This combination of blue light and yellow light produces what may be considered as white light.
However, the white light produced by the LED 100, as well as other similar devices, typically has a core of white light surrounded by an inner ring of blue light, which is further encircled by an outer ring of yellow light. The reason for the color ring may be seen in the emission spectrum 200 of FIG. 2 for LED 100. The emission spectrum 200 portrays that emission energy above 40% exists only in a narrow wavelength band ranging from about 440 nm to 475 nm (i.e., blue light). Furthermore, the emission spectrum 200 exhibits two distinct peaks (one for blue light and one for yellow light at approximately 570 nm) and does not match a typical emission spectrum for white light, which has approximately uniform energy in a wavelength band extending from about 400 to 600 nm.
As another example, U.S. Pat. No. 5,959,316 to Lowery, issued Sep. 28, 1999 and entitled “Multiple Encapsulation of Phosphor-LED Devices,” teaches an LED 18 held in a lead frame 12, as shown in FIG. 3A. A transparent spacer 50 encapsulates the LED 18, and a level of fluorescent material 52 is disposed above the transparent spacer 50. In another embodiment as illustrated in FIG. 3B, a surface-mounted LED light 60 is disposed on a device substrate 62 of a surface mount device. The LED 60 is encapsulated in a transparent spacer 64 which is further covered by a layer of fluorescent material 66 and a final transparent encapsulation layer 68. In both embodiments, the transparent spacer separates the LED from the fluorescent material in an effort to generate more uniform lighting of the fluorescent material to provide a constant, uniform white light LED. Although the yellow color ring problem may be reduced, the corresponding emission spectrum may not approach that of white light with a much broader band of emitted wavelengths. Thus, the quality of the emitted light may be poor, with a corresponding low color rendering index (CRI).
Accordingly, what is needed is a solid state device capable of producing uniform white light with a broad emission spectrum, preferably with a high CRI and without any noticeable color rings.